Atmospheric circulation

Photo by: Hynek Kalista

Atmospheric circulation is the movement of air at all levels of the
atmosphere over all parts of the planet. The driving force behind
atmospheric circulation is solar energy, which heats the atmosphere with
different intensities at the equator, the middle latitudes, and the poles.
The rotation of Earth on its axis and the unequal arrangement of land and
water masses on the planet also contribute to various features of
atmospheric circulation.

Wind cells

There are three wind cells or circulation belts between the equator and
each pole: the trade winds (Hadley cells), prevailing westerlies (Ferrell
cells), and polar easterlies (polar Hadley cells). The trade winds or
Hadley cells are named after the English scientist George Hadley
(1685–1768), who first described them in 1753. As air is heated at
the equator, it rises in the troposphere, the lowest 10 miles (16
kilometers) of Earth's atmosphere. In the wake of the warm rising
air, low pressure develops at the equator. When the air reaches the top of
the troposphere, called the tropopause, it can rise no farther and begins
to move toward the poles, cooling in the process.

At about 30 degrees latitude north and south, the cooled air descends back
to the surface, pushing the air below it toward the equator, since air
flows always move toward areas of low pressure. When the north and south
trade winds meet at the equator and rise again, an area of calm develops
because of the lack of cross-surface winds. Early mariners called this
area the doldrums (from an Old English word meaning dull) because they
feared their sailing ships would be stranded by the lack of wind.

While most of the trade-wind air that sinks at 30 degrees latitude returns
to the equator, some of it flows poleward. At about 60 degrees latitude
north and south, this air mass meets much colder polar air (the areas
where this occurs are known as polar fronts). The warmer air is forced
upward by the colder air to the tropopause, where most of it moves back
toward the equator, sinking at about 30 degrees latitude to continue the
cycle again. These second circulation belts over the middle latitudes
between 30 degrees and 60 degrees are the prevailing westerlies or Ferrell
cells, named after the American meteorologist William Ferrell
(1817–1891), who discovered them in 1856.

Calm regions also occur at 30 degrees latitude where Hadley cells and
Ferrell cells meet because of the lack of lateral wind movement. These
regions were given the name horse latitudes by sailors bringing horses to
the Americas. Stranded by the lack of winds, sailors often ate their
horses as supplies ran low.

The air at the top of polar fronts that does not return toward the equator
moves, instead, poleward. At the poles, this air cools, sinks, and flows
back to 60 degrees latitude north and south. These third circulation belts
over the poles are known as polar easterlies or polar Hadley cells because
they flow in the same direction as the Hadley cells near the equator.
However, they are not as powerful since they lack the solar energy present
at the equator.

Words to Know

Coriolis effect:
Moving object appearing to travel in a curved path over the surface of
a spinning body.

Doldrums:
Region of the equatorial ocean where winds are light and unpredictable.

Horse latitudes:
Region of the oceans around 30 degrees latitude where winds are light
and unpredictable.

Jet stream:
Rapidly moving band of air in the upper atmosphere.

Polar front:
Relatively permanent front formed at the junction of the Ferrell and
polar Hadley cells.

The Coriolis effect

The air flows in these three circulation belts or cells do not move in a
straight north to south or south to north route. Instead, the air flows
seem to move east to west or west to east. This effect was first
identified by the French mathematician Gaspard-Gustave de Coriolis
(1792–1843) in 1835. Coriolis observed that, because of the
spinning of the planet, any moving object above Earth's surface
tends to drift sideways from its course of motion. In the Northern
Hemisphere, this movement is to the right of the course of motion. In the
Southern Hemisphere, it is to the left. As a result, surface winds in
Hadley cells—both in the
equatorial and polar regions—blow from the northeast to the
southwest in the Northern Hemisphere and from the southeast to the
northwest in the Southern Hemisphere. Surface winds in Ferrell cells tend
to blow in the opposite direction: from the southwest to the northeast in
the Northern Hemisphere and from the northwest to the southeast in the
Southern Hemisphere.

Variations and wind patterns

The conditions of the wind cells described above are for general models.
In the real world, actual wind patterns are far more complex. Many
elements play a part in disrupting these patterns from their normal
course, as described by Hadley and Ferrell. Since the Sun does not always
shine directly over the equator, air masses in that area are not heated
equally. While some masses in a cell may be heated quickly, creating a
strong flow upward, others may not receive as much solar energy, resulting
in a much weaker flow. Unevenness in the surface of the planet also
affects the movement of air masses in a cell. A mass moving across a
uniform region, such as an ocean, may be undisturbed. Once it moves over a
region with many variations, such as a mountainous area, it may become
highly disturbed.

Doppler radar used to measure the speed and direction of local winds.
(Reproduced by permission of

National Oceanic Atmospheric Administration

.)

The jet streams

In 1944, an especially dramatic type of atmospheric air movement was
discovered: the jet streams. These permanent air currents are located at
altitudes of 30,000 to 45,000 feet (11 to 13 kilometers) and generally
move with speeds ranging from about 35 to 75 miles (55 to 120 kilometers)
per hour. It is not uncommon, however, for the speed of jet streams to be
as high as 200 miles (320 kilometers) per hour.

These narrow tubes of air, which usually travel west to east, are created
by the great temperature and pressure differences between air masses.
There are four major jet streams, two in each hemisphere. Polar jet
streams, formed along the polar front between the Ferrell and polar Hadley
cells, move between 30 degrees and 70 degrees latitude. The other jet
streams move between 20 degrees and 50 degrees latitude.

Jet streams do not move in straight lines, but in a wavelike manner. They
may break apart into two separate streams and then rejoin, or not. In
winter, because of greater temperature differences, jet streams are
stronger and move toward the equator. In summer, with more uniform
temperatures, they weaken and move poleward. The movement of the jet
streams is an important factor in determining weather conditions in
mid-latitude regions since they can strengthen and move low-pressure
systems.